In a variety of application fields of signal transmission, such as e.g. cable or broadcast radio and television or mobile radio, information-carrying base-band signals are transmitted as band-pass signals (also denoted as Radio Frequency (RF) signals) in higher frequency bands (at RF frequencies). The reasons for RF transmission are manifold: Higher frequency bands are usually more suitable for signal transmission via space or via transmission lines that are shared among several transmitters and receivers; the shift of the frequency band allows for the application of various modulation techniques that can be adapted to the characteristics of the transmission channel (space or cable); the spectrum of the information-carrying base-band signals can be modified by the frequency transformation to allow for a more efficient use of the available transmission bandwidth (spreading or compression of the spectrum); Frequency Division Multiplex (FDM) can be applied by transmitting the information-carrying base-band signals of several information sources in adjacent small RF sub-bands (RF channels), where each RF sub-band has the same bandwidth, but different center frequencies.
In today's digital mobile radio systems, such as for instance the Global System for Mobile Communications (GSM), in general more than one frequency band is available for RF transmission. GSM in Europe uses two frequency bands of 25 MHz bandwidth centered at 900 MHz (GSM900) and 1800 MHz (GSM1800), respectively, wherein each of these frequency bands comprises an up-link (information transfer from mobile station to base station) and a down-link (information transfer from base station to mobile station) frequency band, and wherein each of these up- and down-link bands further comprises a plurality of FDM sub-bands (RF channels) of 200 kHz bandwidth each. In the US, said frequency bands are centered at 850 and 1900 MHz, respectively.
A dual-band European GSM mobile phone is capable of transmitting and receiving in both the GSM900 and GSM1800 band. The basic set-up of a RF transmitter for such a dual-band phone is depicted in FIG. 1.
FIG. 1 shows an I/Q-Filter 1, that low-pass filters the I/Q-modulated base-band signal that is output by a GSM modulator (not shown), yielding a filtered I/Q-modulated base-band signal 2. GSM uses Gaussian Minimum Shift Keying (GMSK) or 8-Phase Shift Keying (8-PSK) as modulation techniques, so that the modulated complex-valued base-band signal can be represented by an Inphase (I) component and Quadrature (Q) component. The transfer of the I/Q-modulated base-band signal to the RF frequency bands with center frequencies of 900 or 1800 MHz is accomplished by the mixers 3-1 and 3-2, respectively. Mixer 3-1 is capable of transferring the I/Q-modulated signal applied to its input to the lower GSM900 band, whereas mixer 3-2 is capable of transferring the I/Q-modulated signal applied to its input to the GSM1800 band. The signal at the outputs of the mixers 3-1 and 3-2 are denoted as transferred signals 4-1 and 4-2, respectively. Note that, in the process of mixing, the I/Q-modulated signal is transferred to the center frequency of the RF sub-band (the RF channel) the mobile phone was assigned to, i.e. to a center frequency in the range 900±25/2 MHz and 1800±25/2 MHz. Each mixer 3-1, 3-2 comprises a Voltage-Controlled Oscillator (VCO) for generating the required RF sub-band center frequency, and a Phase-Locked-Loop (PLL) circuit that controls the VCO in order to accurately maintain said RF sub-band frequency. The bandwidth (around 25 MHz) and center frequency (900 or 1800 MHz) of the frequency band that can be covered by each mixer 3-1, 3-2 is thus defined by the deployed VCO and PLL. The transferred signals 4-1 or 4-2 are then fed into Variable Gain Amplifiers (VGAs) 5-1 and 5-2 for the GSM900 and GSM1800 band, respectively, yielding amplified transferred signals 6-1 and 6-2, respectively. The design of the VGAs 5-1 and 5-2 generally depends on the frequency of the signals that are to be amplified, so that the VGAs 5-1 and 5-2 are different. The amplified transferred signals 6-1 and 6-2 are then fed into power amplifiers 7-1 and 7-2, respectively, and the output signals 8-1 and 8-2 of said power amplifiers 7-1 and 7-2 are transmitted by the transmit antennas 9-1 and 9-2 corresponding to the GSM900 and GSM1800 band, respectively. Note that the design of the power amplifiers 7-1 and 7-2 and the transmit antennas 9-1 and 9-2 depend on the frequency of the input signals as well.